"LOS ANGELES -- An environmental group threatened to sue two of the nation's biggest rail owners Tuesday under a novel legal theory that would classify diesel exhaust as hazardous waste.

The Natural Resources Defense Council sent letters to Union Pacific Corp. and Burlington Northern Santa Fe Railway, saying it will file a lawsuit within 90 days under the Resource Conservation and Recovery Act, which regulates hazardous solid waste disposal.

The notice letter, which is required before proceeding with a lawsuit, cited problems at 16 rail yards across California, from Oakland to San Bernardino.

The conservation group argues that minute particles in diesel air pollution, which include lead, cadmium, arsenic and other toxic elements, are solid waste. If successful, such a suit could open the door for legal action against similar air pollution sources such as ports, airports or anywhere with a lot of diesel equipment, said David Pettit, a senior attorney with the council. "